1. Field of the Invention
This invention generally relates to reaction cassettes and the like used for performing analytical reactions incorporating one or more analytical reagents. More specifically, the present invention relates to controlling the deposition of liquid analytical reagents over discrete reagent zones in such reaction cassettes.
2. Description of the Prior Art
Analytical assay procedures are ubiquitous in a variety of industrial, environmental and medical applications for determination of analytes in sample solutions. Such procedures are employed for determining, inter alia, the amount of an analyte present in a test sample (typically, a liquid reaction mixture or buffer solution) and involve a liquid analytical reaction between the analyte and one or more analytical reagents and which can commonly require a plurality of manipulative steps in order to make the required analytic determination.
Such assay procedures typically involve a number of analytical reactions to be performed sequentially in accordance with a predefined assay protocol and require manipulation of a liquid reaction mixture through a plurality of discrete zones or areas to sequentially contact and react with one or more analytical reagents disposed thereat for realizing various functional steps of the assay.
In realizing self-contained reaction cassettes or vessels particularly adapted to performing analytical assay procedures of the above-noted type, it is important that a specified amount of a liquid reagent be deposited in a controlled manner in specific localized areas in order that the required plurality of discrete reagent zones or areas be defined at appropriate locations within the reaction cassette. Reaction cassettes of this type are provided with a reaction channel which includes the reagent zones disposed at appropriate locations along the channel and adapted to be sequentially contacted by a liquid reaction mixture as it flows along a predefined path in the reaction channel.
The reaction channel is typically defined on a molded plastic substrate having an external surface which is treated to yield a wettable or hydrophilic surface in order to permit the free flow of a liquid therealong. Well-known treatments for rendering such surfaces hydrophilic include plasma treatments such as plasma etching and plasma polymerization, corona discharge, wet chemical treatment, surface coating technologies, and the like.
While the hydrophilic substrate surface permits free flow of the liquid reaction mixture, its very hydrophilic nature poses a significant problem in controlled deposition of liquid analytical reagents at the specific locations where the plurality of reagent zones are defined within the reaction channel. This is because the hydrophilic properties of the surface cause any liquid reagent disposed thereupon to spread in an uncontrollable manner and, accordingly, render the controlled and localized definition of reagent zones extremely difficult.
Attempts have been made to better define the reagent zones by containing the liquid reagent within the zones by means of surface features such as "wells" or "barriers" defined about the confines of the required zones. However, in practical implementations, it becomes necessary that such containment features be extremely shallow in order that the liquid reagents disposed thereupon be capable of being easily contacted and completely removed by the action of a buffer solution, i.e., the liquid reaction mixture, washing over them as the solution flows across the reaction channel where the reagent zones are defined.
These approaches are problematic because the combination of the hydrostatic force produced by the height of the typically single drop of reagent positioned thereupon and the hydrophilic nature of the substrate surface cause the volume of liquid reagent to overcome the necessarily shallow containment barrier and migrate uncontrollably over the reaction channel surface. The controlled deposition of liquid analytical reagent for precise definition of reagent zones is, thus, a difficult, practical aspect of conducting sequential assay procedures.